High performance metal alloys are critical materials for a wide variety of equipment manufactured today. As an example, many molybdenum alloys possess a great deal of high temperature strength. Other types of refractory metal alloys also exhibit desirable combinations of high strength and low thermal conductivity.
X-ray equipment is a good example of the usefulness of high performance alloys. Many components within these devices are made from such materials. Often, the x-ray target and some of the related components are made from molybdenum alloys like titanium-zirconium-molybdenum (TZM). Other components within the device may be made from niobium or tantalum alloys. The utility of these materials is based in large part on their ability to function well in the high temperature environment created during the operation of an x-ray unit.
Obviously, the welds or joints between various metallic structures in a device like an x-ray machine have to be reliable and durable. However, some of the characteristics of high performance alloys can present challenges in obtaining high quality, stable joints. For example, the surface of TZM has the tendency to oxidize to some extent. This oxide, which is difficult to remove, can make bonding to other alloys quite difficult.
Moreover, TZM exhibits limited ductility at room temperature. In assembling the many components of an x-ray device, parts made of TZM will be joined to parts made from more ductile alloys. For example, x-ray tubes which are used in radiology often employ a rotary anode. The "target" is the portion of the anode where the electron beam makes contact, and the x-rays are generated. It is usually shaped like a disk, and is fixed to a support shaft, which is itself connected to a rotor. The anode target is usually made of TZM. It is sometimes joined to an insert made from a ductile, tantalum-based alloy, as disclosed, for example, in U.S. Pat. No. 5,498,186 (M. Benz et al.).
This bonding between metals of varying ductility can prove troublesome during the assembly and operation of an x-ray device. Rotary targets are often exposed to very strong thermal shocks, and they can reach very high temperatures. Failure of x-ray devices in the field has often been traced to connections in this section of the device. In some instances, mechanical stress can loosen the rotary target, and the entire anode assembly can then become unbalanced. Unacceptable vibration and/or mechanical breakage of the assembly may then occur. The need for a balanced target/stem assembly is also critical during the manufacturing cycle, especially in the case of the larger x-ray targets being made today. The frequent occurrence of unbalanced assemblies leads to reduced manufacturing yields.
It should thus be apparent that there is a continuing need for improvements in joining structures made from different metal alloys. More specifically, there would be considerable benefit in new techniques for connecting structures formed from alloys which have different ductility levels, e.g., molybdenum-based alloys joined to structures made from more ductile alloys like those based on tantalum. These techniques should be especially suitable for connecting various x-ray components--especially those used in rotary anode-type x-ray units. Furthermore, these new processes should be compatible with existing fabrication techniques currently being used to manufacture x-ray equipment.